Cryogenic powder making is a relatively new mode of providing a powdered raw material which can be put to use in powder metallurgy techniques and other applications. Cryogenic powder holds great promise because it can provide powdered material at a significantly lower cost and it may result in more useable physical properties, if not enhanced physical properties, for a sintered powdered part.
Essentially cryogenic powder making comprises subjecting scrap metal, or other solid starting metal material, to a temperature below the transition temperature of said metal, such as -(30.degree.-40).degree. F for ferrous based material. The metal becomes so brittle at such depressed temperatures that agitation within a conventional ball mill will reduce the scrap or starting metal material to a powder form over a predetermined period of time and stress from the ball milling elements. At the same time, any oil or other organic materials coating the scrap metal, particularly scrap metal in the form of machine turnings, will also freeze and be removed during the impaction by the ball milling elements; such frozen debris can be screened and separated.
To insure that the scrap metal is in the embrittled condition at the point of impaction, it is necessary to direct a supply of liquid nitrogen against the scrap metal immediately prior to introducing the scrap metal into the mill itself. The comminuted particles resulting from a predetermined amount of ball milling under such embrittled conditions, produces metal particle shapes which are flake-like or irregular, certainly not spherical. The layer-like or flake configuration results from the two facts: (a) the turning was originally ribbon-like, and (b) comminution takes place by fracture.
When such cryogenically produced powder is subjected to conventional powder metallurgy techniques, with a compacted quantity of such powder being heated to a sintering temperature, oxidation of ingredients such as manganese and silicon will typically take place prior to diffusion and completion of the sintering step. Such oxidation results because these elements require more sintering atmosphere control than is normally possible in current, more stringent operations.